Materials that have a unique shape, size, and composition may have innovative functions and are therefore important. In particular, materials of a unique shape, size, and composition that cannot be made artificially have enormous potential for applications. For example, a ceramic material produced by a representative iron bacterium, Leptothrix ochracea, is a sheath-shaped substance with a diameter of about 1 μm and a length of about 200 μm, and the composition of the components other than oxygen is known to have a Fe:Si:P ratio of about 80:15:5. It is also known that the hollow structure of the ceramic material is composed of amorphous nanoparticles with a diameter of 100 nm or less (about 10 to 40 nm) (Non-Patent Document 1).
Ceramic materials produced by iron bacteria, which clog pipes and cause red water, have been only disposed of as waste. However, ceramic materials are worthy of greater attention because they are derived from organisms and thus environmentally friendly, and they mainly consist of the ubiquitous elements iron and silicon and are thus a continuously available unutilized resource. Moreover, any attempt to artificially produce such a unique structure would require a huge amount of time and effort as well as immense technology and energy. Accordingly, the development of a novel material by utilizing a ceramic material derived from nature is highly significant in terms of both of science and technology.
As carriers for enzymes, inorganic materials, such as diatomaceous earth, celite, silica, and glass beads, have been used as is. However, the use of such an inorganic material as is may have problems such as low enzyme loading and enzyme activity impairment. Accordingly, as a material suitable for immobilizing enzymes, the development of an organic-inorganic composite material having a surface modified with an organic group has been progressing. For example, modified kaolinite spherical carriers, modified magnetic nanoparticles, modified gold-silica composite nanoparticles, etc., are known as such materials.
Toyonite-200M, a modified kaolinite spherical carrier, is a material produced by modifying a spherical porous ceramic carrier, Toyonite, obtained by processing kaolinite, with a silane coupling agent. Toyonite-200M can be used for immobilizing an enzyme (Patent Literature (PTL) 1 and Non-patent Literature (NPL) 2).
Modified magnetic nanoparticles are prepared by applying a silane coupling agent to maghemite nanoparticles. Immobilization of the modified magnetic nanoparticles and lipase by a covalent bond is disclosed (Non-Patent Literature (NPL) 3).
Modified gold-silica composite nanoparticles are prepared by self-assembly of silica and gold mediated by a polymeric compound, subsequent sintering treatment, and coordination of the terminal thiol of an organic group on the gold surface. Further, an enzyme is immobilized thereon by a covalent bond with a functional group of the organic group (Non-Patent Document 4).
However, the bond density of the organic group of such known materials is not so high. Accordingly, only about 1 mass %, i.e., a very small amount, of an enzyme can be immobilized thereon. In addition, such known materials have other problems such as an insufficient level of durability of enzyme activity.
Further, for example, lipase, which is an oil and fat hydrolase, can catalyze hydrolysis of an ester bond as well as transesterification and esterification reactions in organic solvents. Further, lipase, which exhibits excellent properties in kinetic optical resolution of racemic compounds, can find a wide variety of application in the fields of organic synthesis and pharmaceuticals. Immobilized lipase comprising diatomaceous earth, celite, Toyonite, or the like as a carrier has already been widely used. However, such an immobilized lipase poses problems such as low enzyme loading, and enzyme activity reduction and enzyme detachment after repeated use. Overcoming these problems would enable repeated use for a longer period using a smaller reactor, resulting in an industrially advantage.